This invention relates to CDMA cellular radio systems, and particularly to controlling the transmit power of base stations in a CDMA system when operating in soft handoff with respect to a mobile station.
FIG. 1 depicts a portion of a conventional CDMA network operating in conjunction with the public switched telephone network (PSTN) 10. Connected to the PSTN 10 is a mobile telephone switching office (MTSO) 20. Connected to MTSO 20 is base station controller (BSC) 30, which in turn connects to base transceiver stations (BTS) 40 and BTS 50. BTS""s 40 and 50 are equipped with antennas 45 and 55 respectively, which transmit and receive radio signals in an area between them and mobile stations (MS) 60. A forward link (47, 57) is defined as transmission from a BTS to a MS. A reverse link (46, 56) is defined as transmission from a MS to a BTS. The area serviced by a BTS comprises a cell.
Depending on a MS""s position within a cell and on the terrestrial features in the vicinity, a MS is assumed to be in communication with one or more BTS""s. In FIG. 1, MS 60 is assumed to be in communication with BTS""s 40 and 50 both, over forward links 47 and 57 respectively, and over reverse links 46 and 56 respectively.
Forward link power control has two aspects: an open loop aspect and a closed loop aspect. The open loop aspect is used to compensate for the effect of distance variations and shadowing which are usually reciprocal on both links. The multipath fading conditions, however, are independent of the forward and reverse links, and so the mobile station has to tell the base station how to adjust its power to compensate fading. When the received energy-per-bit over noise density (Eb/No) from the base station is high (or low), the mobile station sends a power control command (PC) every power control group (also known as a slot) instructing the base station to decrease (or increase) its transmitted power. The power control command is sent at a fixed rate (Tp). Typical rates in third generation (3G) CDMA systems are 800 or 1500 power control commands per second.
A power control command is currently sent once during each xe2x80x9cpower control groupxe2x80x9d(A power control group is also referred to as a xe2x80x9cslotxe2x80x9d). The power control command may be a single bit (one state connoting increase and the other connoting decrease) or it may be a multibit command to allow changing the power by different steps.
Handoff procedures are employed for passing a mobile user from one base station to another. A mobile station is said to undergo a hard handoff if it disconnects its transmission with the current base station and initiates transmission with a new base station. If the mobile station can communicate simultaneously with two or more base stations it is said to be in soft handoff (SHO). Soft handoff is one of the advantages that CDMA cellular systems have over other cellular systems such as frequency-division multiple access (FDMA) since it provides diversity gain and reduces interference which enables the system to support more users. Mobile station 60 of FIG. 1 is in soft handoff, since it is in communication with two BTS""s (40 and 50).
In the conventional scheme, the mobile station sends power control commands at the same rate (i.e. one command during each power control group) when it communicates with a single base station as it does when it is not in soft handoff, as depicted in FIG. 2. This is known as fast power control and is useful for tracking the changes in the communication channel due to multipath fading. A mobile station enters soft handoff when the signals received from at least two base stations are within a certain strength margin, and thus gain is achievable by combining signals (diversity gain). If the power levels of the signals are significantly different, the gain from diversity is lost. Also, if one base station starts transmitting at a high power compared to the power level of the other base station, interference is increased which reduces the system capacity. The base stations will keep transmitting at comparable power levels if the power control commands transmitted by the mobile station can be received without error.
In practice, however, error-free reception of the power control commands is unusual. The power commands are sent uncoded to avoid the delay associated with coding; hence, even when there is a good connection between the mobile station and the BTS, the error is about 5%. If the connection is weaker, the error rate can be significantly higher. Erroneous reception of the power control commands at the base stations can result in their transmitting at markedly different power levels. This results in a loss in the diversity gain that soft handoff can provide. It further can lead to increasing the interference (xe2x80x9cnoise floorxe2x80x9d) perceived by the other mobile stations which reduces the overall capacity of the CDMA system.
In conventional systems, the mobile station uses the same method to issue the power control commands when it is communicating with a single base station (i.e. not in soft handoff) and when it is communicating with more than one base station (i.e., in soft handoff). However, the two situations should be dealt with differently. When the mobile sends a single power control command to several base station, one or more of these base stations may receive this command in error. This results in that base station transmitting at a different power level than the other base stations. This results in a loss of diversity gain that we get from soft handoff and can also increase the interference in the system, which reduces the capacity.
One proposal that has been tried is to balance or synchronize the base stations every x frames by the base station controller 30. This is a slow process that needs the base stations to signal their transmit powers to the base station controller which then will decide what power levels all the base stations should use and signal this back to the base stations. Although this is beneficial, in addition to being slow this process does not address the main cause of the problem which is the power control commands being in error. Such a power synchronization technique is described in U.S. patent application Ser. No. 09/296,974 filed on Apr. 22, 1999 by Chheda et al, and assigned to the same assignee as the present application.
Another proposal suggested in U.S. patent application Ser. No. 09/352,299 filed on Dec. 7, 1999 by Hashem et al. spreads each power control command sent by the mobile station over a plurality of consecutive power control groups (time slots). Although this does alleviate the problem it requires changing the way in which the mobile issues the power control commands which is a standards issue.
The present invention comprises a method and apparatus in a base station for interpreting power control commands sent from the mobile station and received at the base stations while the mobile station is in soft handoff.
Instead of using every received power control command to cause a corresponding adjustment (increase or decrease) in transmit power, the present invention proposes that the number of adjustments be fewer than the number of power control commands sent by the mobile station and received by the base station.
According to one aspect, at least one command in every n consecutive commands is simply ignored. In the preferred embodiment n is 2 and one of the two commands is ignored which means that every other command is ignored.
According to another aspect, every n consecutive commands are interpreted by an algorithm that determines whether the n commands together should be considered an effective power control command. In the simplest case n would be 2 and the algorithm would operate to perform a simple majority vote to indicate an effective power increase command only if both consecutive commands are increase commands and an effective power decrease command only if both consecutive commands are decrease commands. It is not envisaged that n would extend much beyond 2 but n=3 could be feasible with an effective power control command being derived if at least two of the three consecutive received commands were of the same type (i.e. either increase or decrease).
The invention provides more error protection to the power control commands and reduces the rate at which base stations adjust their power. The invention also reduces the average transmission power and the deviation between the transmission powers of the base stations.
The invention will next be described in connection with exemplary embodiments; however, it should be clear to those skilled in the art that various modifications, additions and subtractions can be made without departing from the spirit or scope of the claims.